example in readme changed. todo update

README.md

@@ -69,60 +69,4 @@ Define this for your symbolic types if you want `SymbolicUtils.simplify` to appl
 specific to numbers (such as commutativity of multiplication). Or such
 rules that may be implemented in the future.
 
-## Example
-
-Suppose you were feeling the temptations of type piracy and wanted to make a quick and dirty
-symbolic library built on top of Julia's `Expr` type, e.g.
-
-```julia
-for f ∈ [:+, :-, :*, :/, :^] #Note, this is type piracy!
-    @eval begin
-        Base.$f(x::Union{Expr, Symbol}, y::Number) = Expr(:call, $f, x, y)
-        Base.$f(x::Number, y::Union{Expr, Symbol}) = Expr(:call, $f, x, y)
-        Base.$f(x::Union{Expr, Symbol}, y::Union{Expr, Symbol}) = (Expr(:call, $f, x, y))
-    end
-end
-
-Base.zero(t::Expr) = 0
-
-ex = 1 + (:x - 2)
-```
-
-
-How can we use SymbolicUtils.jl to convert `ex` to `(-)(:x, 1)`? We simply implement `istree`,
-`head`, `arguments` and we'll be able to do rule-based rewriting on `Expr`s:
-```julia
-using TermInterface
-using SymbolicUtils
-
-
-TermInterface.istree(ex::Expr) = ex.head == :call
-TermInterface.operation(ex::Expr) = ex.args[1]
-TermInterface.arguments(ex::Expr) = ex.args[2:end]
-TermInterface.similarterm(x::Type{Expr}, head, args, symtype=nothing; metadata=nothing) = 
-    Expr(:call, head, args...)
-
-TermInterface.issym(s::Symbol) = true
-Base.nameof(s::Symbol) = s
-
-@rule(~x => ~x - 1)(ex)
-```
-
-However, this is not enough to get SymbolicUtils to use its own algebraic simplification system on `Expr`s:
-```julia
-simplify(ex)
-```
-
-The reason that the expression was not simplified is that the expression tree is untyped, so SymbolicUtils 
-doesn't know what rules to apply to the expression. To mimic the behaviour of most computer algebra 
-systems, the simplest thing to do would be to assume that all `Expr`s are of type `Number`:
-
-```julia
-Base.zero(t::Expr) = 0
-TermInterface.symtype(::Expr) = Real
-TermInterface.symtype(::Symbol) = Real
-
-simplify(ex)
-```
-
-Now SymbolicUtils is able to apply the `Number` simplification rule to `Expr`.
+<!-- TODO update examples -->